1. Field of the Invention
The present invention relates generally to position encoders, and more specifically to code track structures for use in position encoders.
2. Discussion of the Prior Art
Optical encoders are typically used to provide positional information for rotating shafts. In a typical arrangement, the disk is attached to a rotating shaft. The disk includes code elements which are organized as data tracks to provide information about the angular position of the shaft. The code elements are positioned circumferentially on the disk. The disk can be opaque, while the code elements are generally transparent, or vice versa.
Adjacent to the rotating disk are detection circuits which are fixed with respect to the disk and used to read the code elements as they pass by the detection circuits. The detection circuits usually include a light source and photodetector, and sometimes a mask. The disk is positioned between a light source and a photodetector. A mask is sometimes used to set the aperture through which light passing through the code disk is examined by the photodetector. Typically, a mask will have a reverse image of the pattern sought to be detected. Registration is established between the mask and the pattern sought to be detected when the patterns on the code elements and mask match. At this point, light which is being transmitted from the source to the photodetector is blocked causing a low or zero light signal from the photodetector. When the disk and mask are out of registration, varying levels of light pass through the disk and mask on to the photodetector, thus generating a signal having a magnitude which indicates the amount of light incident upon the photodetector.
In incremental encoders, the detection means count the number of code elements which pass in registration when the disk is rotated with the shaft, so that the total number of elements counted from a reference point is proportional to the angular position through which the disk, hence the shaft, has been rotated.
In incremental encoders, the reference or index signal is usually generated by a different track, also arranged circumferentially on the disk, generally termed an index track.
In a typical arrangement, the index track is a single, circular path on the disk, separate from the data track(s), which is either entirely transparent or entirely opaque except for an single index mark positioned transversely across one point of the path. This mark totally blocks (in the case of a transparent path) or totally exposes (in the case of an opaque path) light to the photodetector to the light beam once per revolution of the code disk. In other words, once per revolution of the disk, the index track generates a step change pulse output signal, but the remainder of each revolution is otherwise steady at a constant level signal. In a variation on this technique, two index tracks are provided, one being opaque with a transparent mark, the other being transparent with a coincidental opaque mark to separate a differential index signal. See, for example, LaPlante, U.S. Pat. No. 4,602,155, issued July 22, 1986, and LaPlante, U.S. Pat. No. 4,678,908, issued July 7, 1987. In sophisticated systems, the index mark is generated using a pattern of elements or a sequence of patterns of elements to increase the certainty with which the index mark can be identified and detected. However, these approaches require large area detectors and more complex index tracks.
The method of constructing the type of optical encoders discussed above typically involves depositing a layer of opaque or reflective material on a transparent substrate base.
Another method for providing an index signal is to have a protrusion emanating from the outer periphery of the disk. However, such an arrangement is both fragile and dangerous.
For low cost encoders, it would be desirable to fabricate the code disk out of metal, with the code elements for data and index tracks being formed by electroforming, depositing, etching or stamping clear through the metal. However, when an index track is desired having an opaque index mark with the rest of the track being clear, the formation of the clear portion of the track would result in removal of structural support for the outer, code track supporting portions of the disk.